Work vehicle and method for controlling same

ABSTRACT

A work vehicle is equipped with an engine, a hydraulic pump, a work implement, a travel device, an accelerator operating member, a work implement operating member, and a control unit. The hydraulic pump is driven by the engine. The work implement is driven by hydraulic fluid discharged from the hydraulic pump. The travel device is driven by the engine. The accelerator operating member changes the engine rotation speed. The work implement operating member operates the work implement. The control unit causes the speed of the work implement to increase by causing the engine rotation speed to increase when an operation amount of the work implement operating member is increased.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National stage application of InternationalApplication No. PCT/JP2014/083259, filed on Dec. 16, 2014. This U.S.National stage application claims priority under 35 U.S.C. §119(a) toJapanese Patent Application No. 2013-260523, filed in Japan on Dec. 17,2013, the entire contents of which are hereby incorporated herein byreference.

BACKGROUND Field of the Invention

The present invention relates to a work vehicle and method forcontrolling the work vehicle.

A hydraulic pump in a work vehicle is driven by driving power from anengine. A work implement is driven by hydraulic fluid discharged fromthe hydraulic pump. However, the driving power from the engine is alsodistributed to a travel device. As a result, the work vehicle travelsdue to the travel device being driven. Moreover, the engine rotationspeed can be changed by an operator operating an accelerator operatingmember, such as an accelerator pedal (see, for example, JapaneseLaid-open Patent 2004-144254).

The speed of the work implement is determined in accordance with theflow rate of hydraulic fluid supplied to the work implement. When theoperator operates a work implement operating member, such as a workimplement lever, the opening surface area of a work implement controlvalve is changed. As a result, the flow rate of the hydraulic fluidsupplied to the work implement is changed. Moreover, in order to allowthe hydraulic pump to be driven by the engine, the flow rate of thehydraulic fluid is also changed due to the engine rotation speed beingchanged. Therefore, the operator is able to adjust the speed of the workimplement by operating the work implement operating member and theaccelerator operating member.

SUMMARY

To increase the speed of the work implement, the operator, for example,tilts the work implement lever further and also steps on the acceleratorpedal. As a result, the opening surface area of the work implementcontrol valve is increased and the engine rotation speed is increasedwhereby the speed of the work implement can be increased. However,because the driving power from the engine is also distributed to thetravel device, the vehicle speed increases when the engine rotationspeed increases. As a result, the operator adjusts the speed of the workimplement while suppressing the increase in the vehicle speed byoperating a brake operating member, such as a brake pedal.

Consequently, the operator adjusts the speed of the work implement andthe vehicle speed by operating the work implement operating member, theaccelerator operating member, and the brake operating member at the sametime. However, this type of complicated operation requires a high levelof technique and is not easy.

An object of the present invention is to provide a work vehicle that isable to maintain balance between the speed of the work implement and thevehicle speed with a simple operation, and a method for controlling thework vehicle.

A work vehicle according to a first exemplary embodiment of the presentinvention is equipped with an engine, a hydraulic pump, a workimplement, a travel device, an accelerator operating member, a workimplement operating member, and a control unit. The hydraulic pump isdriven by the engine. The work implement is driven by hydraulic fluiddischarged from the hydraulic pump. The travel device is driven by theengine. The accelerator operating member is a member for an operator tochange the engine rotation speed. The work implement operating member isa member for operating the work implement. The control unit causes thespeed of the work implement to increase by causing the engine rotationspeed to increase when an operation amount of the work implementoperating member is increased.

The work vehicle according to the present exemplary embodiment enablesthe engine rotation speed to be increased automatically by the controlunit when an operator operates the work implement operating member. As aresult, the operator is able to adjust the speed of the work implementby operating the work implement operating member without operating theaccelerator operating member. As a result, the speed of the workimplement and the vehicle speed can be adjusted with a simple operation.

The work vehicle is preferably further equipped with a powertransmission device. The power transmission device has an input shaft,an output shaft and a motor and transmits driving power from the engineto the travel device. The power transmission device is configured tochange a rotation speed ratio of the output shaft with respect to theinput shaft by changing the rotation speed of the motor. The controlunit controls the tractive force of the vehicle by controlling theoutput torque of the motor.

In this case, an increase in the tractive force of the vehicle can besuppressed due to the control unit controlling the output torque of themotor even when the engine rotation speed is increased due to anincrease in the speed of the work implement. As a result, the operatoris able to adjust the speed of the work implement by operating the workimplement operating member without performing an operation to suppressthe increase in the vehicle speed. As a result, the speed of the workimplement and the vehicle speed can be adjusted with a simple operation.

The control unit preferably determines a required tractive force whichis a target tractive force of the travel device on the basis of theoperation amount of the accelerator operating member and controls theoutput torque of the motor so that the tractive force of the vehiclebecomes the required tractive force. In this case, the vehicle speed canbe adjusted in response to the operation amount of the acceleratoroperating member even when the engine rotation speed increases due to anincrease in the speed of the work implement.

The control unit preferably causes a discharge capacity of the hydraulicpump to increase in response to an increase in the operation amount ofthe work implement operating member when the operation amount of thework implement operating member is equal to or less than a predeterminedoperation amount. The discharge capacity of the hydraulic pump becomesthe maximum capacity when the operation amount of the work implementoperating member is the predetermined operation amount. The control unitcauses the engine rotation speed to be increased in response to anincrease in the operation amount of the work implement operating memberwhen the operation amount of the work implement operating member isgreater than the predetermined operation amount.

The capacity in the present description signifies the amount ofhydraulic fluid discharged for each one rotation of the hydraulic pump.Moreover, the flow rate signifies the amount of hydraulic fluiddischarged by the hydraulic pump per unit of time.

In this case, the speed of the work implement is controlled bycontrolling the discharge capacity of the hydraulic pump until thedischarge capacity of the hydraulic pump becomes the maximum capacity.The speed of the work implement is controlled with the engine rotationspeed when the discharge capacity of the hydraulic pump reaches themaximum capacity. Accordingly, fuel consumption can be improved.

The control unit preferably has a storage unit for storing required flowrate information which defines the relationship between the operationamount of the work implement operating member and the required flow rateto the hydraulic pump. The control unit determines the required flowrate corresponding to the operation amount of the work implementoperating member by referring to the required flow rate information. Thecontrol unit determines the engine rotation speed on the basis of therequired flow rate and the discharge capacity of the hydraulic pump. Inthis case, the engine rotation speed can be determined so that the speedof the work implement that corresponds to the operation amount of thework implement operating member is obtained.

The work vehicle is preferably further provided with a work implementcontrol valve and a capacity control device. The work implement controlvalve controls the hydraulic pressure supplied to the work implement.The capacity control device includes a load sensing valve and controlsthe discharge capacity of the hydraulic pump so that a differentialpressure between the discharge pressure of the hydraulic pump and theoutlet hydraulic pressure of the work implement control valve becomes apredetermined value. When the discharge capacity of the hydraulic pumpis controlled by the flow rate control device having the load sensingvalve, the differential pressure of the work implement control valveneeds to be compensated. The control unit as described above determinesthe engine rotation speed on the basis of the required flow rate and thedischarge capacity of the hydraulic pump whereby the rotation speed ofthe engine can be determined in consideration of the required flow rateand an increase output amount for compensating the differential pressureof the work implement control valve.

The control unit preferably causes the required tractive force to fallbelow a value determined on the basis of the operation amount of theaccelerator operating member when the engine rotation speed is increasedin response to the increase in the operation amount of the workimplement operating member. In this case, a behavior similar to aconventional work vehicle can be realized. That is, the driving powerdistributed to the travel device is reduced by increasing the drivingpower distributed to the hydraulic pump when the operator operates thework implement operating member in order to increase the speed of thework implement in the conventional work vehicle. As a result, thebehavior of a reduction in the tractive force is brought about in thevehicle when the operator operates the work implement operating member.A sense of discomfort by the operator can be suppressed in the workvehicle according to the present embodiment by bringing about thebehavior similar to the conventional work vehicle in this way.

The control unit preferably causes the required tractive force to bereduced so that the tractive force is further reduced in comparison to atractive force before the operation of the work implement operatingmember. In this case, a sense of discomfort by the operator can besuppressed by bringing about the behavior similar to a conventional workvehicle.

The power transmission device preferably further includes a planetarygear mechanism. The control unit preferably causes the required tractiveforce to be reduced so that the tractive force is maintained regardlessof the operation amount of the work implement operating member. Thetractive force can be increased slightly due to inertia of the planetarygear mechanism connected to the engine when the engine rotation speed isincreased in the work vehicle provided with the planetary gearmechanism. Therefore, the tractive force is greater in comparison towhen the speed control is not performed when the engine rotation speedis increased due to speed control of the work implement being performedby controlling the engine rotation speed. Such an increase in thetractive force may impart a sense of discomfort to the operator.

Accordingly, by reducing the required tractive force so that thetractive force is maintained regardless of an operation of the workimplement operating member, the tractive force is maintained at theamount before the operation of the work implement operating member evenwhen the speed control of the work implement is performed by controllingthe engine rotation speed. As a result, a sense of discomfort for theoperator can be suppressed.

The control unit preferably causes the required tractive force to bereduced by multiplying the required tractive force by a predeterminedreduction rate. In this case, the required tractive force can be reducedeasily.

A control method for a work vehicle according to another exemplaryembodiment of the present invention is a control method for a workvehicle equipped with an engine, a hydraulic pump, a work implement, atravel device, an accelerator operating member, and a work implementoperating member. The hydraulic pump is driven by the engine. The workimplement is driven by hydraulic fluid discharged from the hydraulicpump. The travel device is driven by the engine. The acceleratoroperating member is a member for an operator to change the enginerotation speed. The work implement operating member is a member foroperating the work implement. The control method according to thepresent exemplary embodiment includes a step for causing the speed ofthe work implement to increase by causing the engine rotation speed toincrease when an operation amount of the work implement operating memberis increased.

The control method for the work vehicle according to the presentexemplary embodiment allows the engine rotation speed to be increasedautomatically when an operator operates the work implement operatingmember. As a result, the operator is able to adjust the speed of thework implement by operating the work implement operating member withoutoperating the accelerator operating member. As a result, the speed ofthe work implement and the vehicle speed can be adjusted with a simpleoperation.

A work vehicle that is able to maintain balance between the speed of thework implement and the vehicle speed with a simple operation, and amethod for controlling the work vehicle can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a work vehicle according to an exemplaryembodiment.

FIG. 2 is a schematic view of a configuration of the work vehicle.

FIG. 3 is a schematic view of a configuration of a power transmissiondevice.

FIG. 4 illustrates changes in rotation speeds of a first motor and asecond motor with respect to the vehicle speed.

FIG. 5 is a block diagram illustrating a process for determining commandtorques for the motors.

FIG. 6 is a block diagram illustrating processing by a transmissionrequirement determination unit.

FIG. 7 is a block diagram illustrating processing by a required throttledetermination unit.

FIG. 8 is a graph illustrating a relationship between an operationamount and a discharge flow rate of a work implement pump.

FIGS. 9A-9E are timing charts illustrating changes in parameters whencontrolling the speed of the work implement.

FIGS. 10A-10E are timing charts illustrating changes in parameters whencontrolling the speed of the work implement according to anotherexemplary embodiment.

FIG. 11 is a schematic view illustrating a power transmission deviceaccording to a first modified example.

FIG. 12 is a schematic view illustrating a power transmission deviceaccording to a second modified example.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention will be explained indetail with reference to the figures. FIG. 1 is a side view of a workvehicle 1 according to an exemplary embodiment of the present invention.As illustrated in FIG. 1, the work vehicle 1 is equipped with a vehiclebody frame 2, a work implement 3, traveling wheels 4 and 5, and anoperating cabin 6. The work vehicle 1 is a wheel loader and travels dueto the traveling wheels 4 and 5 being rotated and driven. The workvehicle 1 is able to carry out work, such as excavation, by using thework implement 3.

The work implement 3 and the traveling wheels 4 and 5 are attached tothe vehicle body frame 2. The work implement 3 is driven by hydraulicfluid from a below-mentioned work implement pump 23 (see FIG. 2). Thework implement 3 has a boom 11 and a bucket 12. The boom 11 is mountedon the vehicle body frame 2. The work implement 3 includes a liftcylinder 13 and a bucket cylinder 14. The lift cylinder 13 and thebucket cylinder 14 are hydraulic cylinders. One end of the lift cylinder13 is attached to the vehicle body frame 2. The other end of the liftcylinder 13 is attached to the boom 11. The boom 11 swings up and downdue to the extension and contraction of the lift cylinder 13 withhydraulic fluid from the work implement pump 23. The bucket 12 isattached to the tip of the boom 11. One end of the bucket cylinder 14 isattached to the vehicle body frame 2. The other end of the bucketcylinder 14 is attached to the bucket 12 via a bell crank 15. The bucket12 swings up and down due to the extension and contraction of the bucketcylinder 14 with hydraulic fluid from the work implement pump 23.

The operating cabin 6 is attached to the vehicle body frame 2. Theoperating cabin 6 is mounted on the vehicle body frame 2. A seat for theoperator and a below-mentioned operating device are disposed in theoperating cabin 6. The vehicle body frame 2 has a front frame 16 and arear frame 17. The front frame 16 and the rear frame 17 are attached toeach other in a manner that allows swinging in the left-right direction.

The work vehicle 1 has a steering cylinder 18. The steering cylinder 18is attached to the front frame 16 and the rear frame 17. The steeringcylinder 18 is a hydraulic cylinder. The work vehicle 1 is able tochange the advancing direction to the right and left with the extensionand contraction of the steering cylinder 18 due to hydraulic fluid froma below-mentioned steering pump 30.

FIG. 2 is a schematic view of a configuration of the work vehicle 1. Asillustrated in FIG. 2, the work vehicle 1 is equipped with an engine 21,a power take-off device 22 (referred to below as a “PTO 22”), a powertransmission device 24, a travel device 25, an operating device 26, anda control unit 27.

The engine 21 is, for example, a diesel engine. The output of the engine21 is controlled by adjusting the amount of fuel injected into thecylinders of the engine 21. The adjustment of the amount of fuel isperformed by the control unit 27 controlling a fuel injection device 28attached to the engine 21. The work vehicle 1 is equipped with an enginerotation speed detecting unit 31. The engine rotation speed detectingunit 31 detects the engine rotation speed and transmits a detectionsignal indicating the engine rotation speed to the control unit 27.

The work vehicle 1 has the work implement pump 23, the steering pump 30,and a transmission pump 29. The work implement pump 23, the steeringpump 30, and the transmission pump 29 are hydraulic pumps. The PTO 22(power take-off) transmits a portion of the driving power from theengine 21 to the hydraulic pumps 23, 30, and 29. That is, the PTO 22distributes the driving power from the engine 21 to the powertransmission device 24 and the hydraulic pumps 23, 30, and 29.

The work implement pump 23 is driven by driving power from the engine21. The hydraulic fluid discharged from the work implement pump 23 issupplied to the lift cylinder 13 and the bucket cylinder 14 through awork implement control valve 41. The work vehicle 1 is equipped with awork implement pump pressure detecting unit 32. The work implement pumppressure detecting unit 32 detects a discharge pressure (referred tobelow as “work implement pump pressure”) of hydraulic fluid from thework implement pump 23 and transmits a detection signal indicating thework implement pump pressure to the control unit 27.

The work implement pump 23 is a variable displacement hydraulic pump.The discharge capacity of the work implement pump 23 is changed bychanging the tilt angle of a skew plate or an inclined shaft of the workimplement pump 23. A first capacity control device 42 is connected tothe work implement pump 23. The first capacity control device 42 iscontrolled by the control unit 27 and changes the tilt angle of the workimplement pump 23. As a result, the discharge capacity of the workimplement pump 23 is controlled by the control unit 27. The work vehicle1 is equipped with a first tilt angle detecting unit 33. The first tiltangle detecting unit 33 detects the tilt angle of the work implementpump 23 and transmits a detection signal indicating the tilt angle tothe control unit 27.

The first capacity control device 42 has a load sensing valve 46(referred to below as “LS valve 46”). The LS valve 46 controls thedischarge capacity of the work implement pump 23 so that a differentialpressure between a discharge pressure of the work implement pump 23 andan outlet hydraulic pressure of the work implement control valve 41becomes a predetermined value. Specifically, the maximum outlethydraulic pressure between the outlet hydraulic pressure to the liftcylinder 13 and the outlet hydraulic pressure to the bucket cylinder 14is inputted to the LS valve 46. The LS valve 46 controls the dischargecapacity of the work implement pump 23 so that a differential pressurebetween the discharge pressure of the work implement pump 23 and themaximum outlet hydraulic pressure becomes a predetermined value.

The steering pump 30 is driven by driving power from the engine 21. Thehydraulic fluid discharged from the steering pump 30 is supplied to theabove-mentioned steering cylinder 18 through a steering control valve43. The work vehicle 1 is equipped with a steering pump pressuredetecting unit 34. The steering pump pressure detecting unit 34 detectsthe discharge pressure (referred to below as “steering pump pressure”)of hydraulic fluid from the steering pump 30 and transmits a detectionsignal indicating the steering pump pressure to the control unit 27.

The steering pump 30 is a variable displacement hydraulic pump. Thedischarge capacity of the steering pump 30 is changed by changing thetilt angle of a skew plate or an inclined shaft of the steering pump 30.A second capacity control device 44 is connected to the steering pump30. The second capacity control device 44 is controlled by the controlunit 27 and changes the tilt angle of the steering pump 30. As a result,the discharge capacity of the steering pump 30 is controlled by thecontrol unit 27. The work vehicle 1 is equipped with a second tilt angledetecting unit 35. The second tilt angle detecting unit 35 detects thetilt angle of the steering pump 30 and transmits a detection signalindicating the tilt angle to the control unit 27.

The transmission pump 29 is driven by driving power from the engine 21.The transmission pump 29 is a fixed displacement hydraulic pump.Hydraulic fluid discharged from the transmission pump 29 is supplied toclutches CF, CR, CL, and CH of the power transmission device 24 viabelow-mentioned clutch control valves VF, VR, VL, and VH.

The PTO 22 transmits a portion of the driving power from the engine 21to the power transmission device 24. The power transmission device 24transmits the driving power from the engine 21 to the travel device 25.The power transmission device 24 changes the speed of the driving powerfrom the engine 21 and outputs it. An explanation of the configurationof the power transmission device 24 is provided in detail below.

The travel device 25 has an axle 45 and the traveling wheels 4 and 5.The axle 45 transmits driving power from the power transmission device24 to the traveling wheels 4 and 5. As a result, the traveling wheels 4and 5 rotate. The work vehicle 1 is equipped with a vehicle speeddetecting unit 37. The vehicle speed detecting unit 37 detects therotation speed (referred to below as “output rotation speed”) of anoutput shaft 63 of the power transmission device 24. The output rotationspeed corresponds to the vehicle speed and consequently the vehiclespeed detecting unit 37 detects the vehicle speed by detecting theoutput rotation speed. The vehicle speed detecting unit 37 detects therotating direction of the output shaft 63. The rotating direction of theoutput shaft 63 corresponds to the traveling direction of the workvehicle 1 and consequently the vehicle speed detecting unit 37 functionsas a traveling direction detecting unit that detects the travelingdirection of the work vehicle 1 by detecting the rotating direction ofthe output shaft 63. The vehicle speed detecting unit 37 transmitsdetection signals indicating the output rotation speed and the rotatingdirection to the control unit 27.

The operating device 26 is operated by an operator. The operating device26 has an accelerator operating device 51, a work implement operatingdevice 52, a speed change operating device 53, a forward/reverse traveloperating device 54 (referred to below as “FR operating device 54”), asteering operating device 57, and a brake operating device 58.

The accelerator operating device 51 has an accelerator operating member51 a and an accelerator operation detecting unit 51 b. The acceleratoroperating member 51 a is operated to set a target rotation speed of theengine 21. The rotation speed of the engine 21 is changed due to theaccelerator operating member 51 a being operated. The acceleratoroperation detecting unit 51 b detects an operation amount (referred tobelow as “accelerator operation amount”) of the accelerator operatingmember 51 a. The accelerator operation detecting unit 51 b transmits adetection signal indicating the accelerator operation amount to thecontrol unit 27.

The work implement operating device 52 has a work implement operatingmember 52 a and a work implement operation detecting unit 52 b. The workimplement operating member 52 a is operated to actuate the workimplement 3. The work implement operation detecting unit 52 b detects aposition of the work implement operating member 52 a. The work implementoperation detecting unit 52 b outputs a detection signal indicating theposition of the work implement operating member 52 a to the control unit27. The work implement operation detecting unit 52 b detects anoperation amount of the work implement operating member 52 a (referredto below as “work implement operation amount”) by detecting a positionof the work implement operating member 52 a.

The speed change operating device 53 has a speed change operating member53 a and a speed change operation detecting unit 53 b. The operator isable to select a speed range of the power transmission device 24 byoperating the speed change operating member 53 a. The speed changeoperation detecting unit 53 b detects a position of the speed changeoperating member 53 a. The position of the speed change operating member53 a corresponds to a plurality of speed ranges, such as a first speedand a second speed and the like. The speed change operation detectingunit 53 b outputs a detection signal indicating the position of thespeed change operating member 53 a to the control unit 27.

The FR operating device 54 has a forward/reverse travel operating member54 a (referred to below as “FR operating member 54 a”) and aforward/reverse travel position detecting unit 54 b (referred to belowas a “FR position detecting unit 54 b”). The operator can switch betweenforward and reverse travel of the work vehicle 1 by operating the FRoperating member 54 a. The FR operating member 54 a is selectivelyswitched between a forward travel position (F), a neutral position (N),and a reverse travel position (R). The FR position detecting unit 54 bdetects a position of the FR operating member 54 a. The FR positiondetecting unit 54 b outputs a detection signal indicating the positionof the FR operating member 54 a to the control unit 27.

The steering operating device 57 has a steering operating member 57 a.The steering operating device 57 drives the steering control valve 43 bysupplying pilot hydraulic pressure based on an operation of the steeringoperating member 57 a to the steering control valve 43. The steeringoperating device 57 may drive the steering control valve 43 byconverting an operation of the steering operating member 57 a to anelectrical signal. The operator is able to change the travel directionof the work vehicle 1 to the right or left by operating the steeringoperating member 57 a.

The brake operating device 58 has a brake operating member 58 a and abrake operation detecting unit 58 b. The operator is able to operate abraking force of the work vehicle 1 by operating the brake operatingmember 58 a. The brake operation detecting unit 58 b detects anoperation amount of the brake operating member 58 a (referred to belowas “brake operation amount”). The brake operation detecting unit 58 boutputs a detection signal indicating the brake operation amount to thecontrol unit 27. The pressure of the brake oil may be used as the brakeoperation amount.

The control unit 27 has a calculation device, such as a CPU, and amemory, such as a RAM or a ROM, and conducts processing for controllingthe work vehicle 1. The control unit 27 has a storage unit 56. Thestorage unit 56 stores programs and data for controlling the workvehicle 1.

The control unit 27 transmits a command signal indicating a commandthrottle value to the fuel injection device 28 so that a target rotationspeed of the engine 21 is achieved in accordance with the acceleratoroperation amount. The control of the engine 21 by the control unit 27 isdescribed in detail below.

The control unit 27 controls hydraulic pressure supplied to thehydraulic cylinders 13 and 14 by controlling the work implement controlvalve 41 on the basis of the detection signals from the work implementoperation detecting unit 52 b. As a result, the hydraulic cylinders 13and 14 expand or contract to actuate the work implement 3.

Specifically, the storage unit 56 stores work implement control valvecommand value information which defines the relationship between thework implement operation amount and a command current value to the workimplement control valve 41. For example, the work implement controlvalve command value information is a map which defines the relationshipbetween the work implement operation amount and the command currentvalue to the work implement control valve 41. The work implement controlvalve command value information may be a table or a formula or inanother format other than a map. The opening surface area of the workimplement control valve 41 is determined in response to the commandcurrent value. The work implement control valve command valueinformation defines the command current value so that the openingsurface area of the work implement control valve 41 increases incorrespondence to an increase in the work implement operation amount.The control unit 27 refers to the work implement control valve commandvalue information to determine the command current value to the workimplement control valve 41 from the work implement operation amount.

The control unit 27 controls the power transmission device 24 on thebasis of the detection signals from each of the detecting units. Thecontrol of the power transmission device 24 by the control unit 27 isdescribed in detail below.

Next, a detailed explanation of the configuration of the powertransmission device 24 is provided. FIG. 3 is a schematic view of aconfiguration of the power transmission device 24. As illustrated inFIG. 3, the power transmission device 24 is provided with an input shaft61, a gear mechanism 62, the output shaft 63, a first motor MG1, asecond motor MG2, and a capacitor 64. The input shaft 61 is connected tothe above-mentioned PTO 22. The rotation from the engine 21 is inputtedto the input shaft 61 via the PTO 22. The gear mechanism 62 transmitsthe rotation of the input shaft 61 to the output shaft 63. The outputshaft 63 is connected to the above-mentioned travel device 25 andtransmits the rotation from the gear mechanism 62 to the above-mentionedtravel device 25.

The gear mechanism 62 is a mechanism for transmitting driving power fromthe engine 21. The gear mechanism 62 is configured so that the rotationspeed ratio of the output shaft 63 with respect to the input shaft 61 ischanged in response to changes in the rotation speeds of the motors MG1and MG2. The gear mechanism 62 has a FR switch mechanism 65 and a speedchange mechanism 66.

The FR switch mechanism 65 has a forward travel clutch CF (referred tobelow as “F-clutch CF”), a reverse travel clutch CR (referred to belowas “R-clutch CR”), and various other gears not illustrated. The F-clutchCF and the R-clutch CR are hydraulic clutches and hydraulic fluid issupplied from the transmission pump 29 to the clutches CF and CR. Thehydraulic fluid for the F-clutch CF is controlled by an F-clutch controlvalve VF. The hydraulic fluid for the R-clutch CR is controlled by anR-clutch control valve VR. The clutch control valves VF and VR arecontrolled by command signals from the control unit 27.

The direction of the rotation outputted from the FR switch mechanism 65is switched due to the switching between connected/disconnected statesof the F-clutch CF and disconnected/connected states of the R-clutch CR.Specifically, the F-clutch CF is connected and the R-clutch CR isdisconnected when the vehicle is traveling forward. The F-clutch CF isdisconnected and the R-clutch CR is connected when the vehicle istraveling in reverse.

The speed change mechanism 66 has a transmission shaft 67, a firstplanetary gear mechanism 68, a second planetary gear mechanism 69, aHi/Lo switch mechanism 70, and an output gear 71. The transmission shaft67 is coupled to the FR switch mechanism 65. The first planetary gearmechanism 68 and the second planetary gear mechanism 69 are disposed onthe same axis as the transmission shaft 67.

The first planetary gear mechanism 68 has a first sun gear S1, aplurality of first planet gears P1, a first carrier C1 that supports theplurality of first planet gears P1, and a first ring gear R1. The firstsun gear S1 is coupled to the transmission shaft 67. The plurality offirst planet gears P1 mesh with the first sun gear S1 and are supportedin a rotatable manner by the first carrier C1. A first carrier gear Gc1is provided on an outer peripheral part of the first carrier C1. Thefirst ring gear R1 meshes with the plurality of first planet gears P1and is able to rotate. A first ring outer periphery gear Gr1 is providedon the outer periphery of the first ring gear R1.

The second planetary gear mechanism 69 has a second sun gear S2, aplurality of second planet gears P2, a second carrier C2 that supportsthe plurality of second planet gears P2, and a second ring gear R2. Thesecond sun gear S2 is coupled to the first carrier C1. The plurality ofsecond planet gears P2 mesh with the second sun gear S2 and aresupported in a rotatable manner by the second carrier C2. The secondring gear R2 meshes with the plurality of second planet gears P2 and isable to rotate. A second ring outer periphery gear Gr2 is provided onthe outer periphery of the second ring gear R2. The second ring outerperiphery gear Gr2 meshes with the output gear 71, and the rotation ofthe second ring gear R2 is outputted to the output shaft 63 via theoutput gear 71.

The Hi/Lo switch mechanism 70 is a mechanism for switching the drivingpower transmission path of the power transmission device 24 between ahigh-speed mode (Hi mode) in which the vehicle speed is high and alow-speed mode (Lo mode) in which the vehicle speed is low. The Hi/Loswitch mechanism 70 has an H-clutch CH that is connected during the Himode and an L-clutch CL that is connected during the Lo mode. TheH-clutch CH engages or disengages the first ring gear R1 and the secondcarrier C2. The L-clutch CL engages or disengages the second carrier C2and a fixed end 72 to prohibit or allow the rotation of the secondcarrier C2.

The clutches CH and CL are hydraulic clutches, and hydraulic fluid fromthe transmission pump 29 is supplied to each of the clutches CH and CL.The hydraulic fluid for the H-clutch CH is controlled by an H-clutchcontrol valve VH. The hydraulic fluid for the L-clutch CL is controlledby an L-clutch control valve VL. The clutch control valves VH and VL arecontrolled by command signals from the control unit 27.

The first motor MG1 and the second motor MG2 function as drive motorsthat generate driving power using electrical energy. The first motor MG1and the second motor MG2 also function as generators that use inputteddriving power to generate electrical energy. The first motor MG1functions as a generator when a command signal from the control unit 27is applied to activate torque in the reverse direction of the rotatingdirection of the first motor MG1. A first motor gear Gm1 is fixed to theoutput shaft of the first motor MG1 and the first motor gear Gm1 mesheswith the first carrier gear Gc1. A first inverter I1 is connected to thefirst motor MG1 and a command signal for controlling the motor torque ofthe first motor MG1 is applied to the first inverter I1 from the controlunit 27.

The second motor MG2 is configured in the same way as the first motorMG1. A second motor gear Gm2 is fixed to the output shaft of the secondmotor MG2 and the second motor gear Gm2 meshes with the first ring outerperiphery gear Gr1. A second inverter I2 is connected to the secondmotor MG2 and a command signal for controlling the motor torque of thesecond motor MG2 is applied to the second inverter I2 from the controlunit 27.

The capacitor 64 functions as an energy reservoir unit for storingenergy generated by the motors MG1 and MG2. That is, the capacitor 64stores electrical power generated by the motors MG1 and MG2 when thetotal electrical power generation amount of the motors MG1 and MG2 ishigh. The capacitor 64 releases electrical power when the totalelectrical power consumption amount of the motors MG1 and MG2 is high.That is, the motors MG1 and MG2 are driven by electrical power stored inthe capacitor 64. Alternatively, the motors MG1 and MG2 can drive usingthe electrical power stored in the capacitor 64. A battery may be usedin place of a capacitor.

The control unit 27 receives detection signals from the variousdetecting units and applies command signals for indicating the commandtorques for the motors MG1 and MG2 to inverters I1 and I2. The controlunit 27 may output rotation speed commands to the motors MG1 and MG2. Inthis case, the inverters I1 and I2 control the motors MG1 and MG2 bycalculating command torques corresponding to the rotation speedcommands. The control unit 27 also applies command signals forcontrolling the clutch hydraulic pressure of the clutches CF, CR, CH,and CL to the clutch control valves VF, VR, VH, and VL. As a result, thespeed change ratio and the output torque of the power transmissiondevice 24 are controlled. The following is an explanation of theoperations of the power transmission device 24.

An outline of operations of the power transmission device 24 when thevehicle speed increases from zero in the forward travel side while therotation speed of the engine 21 remains fixed, will be explained withreference to FIG. 4. FIG. 4 illustrates the rotation speeds of themotors MG1 and MG2 with respect to the vehicle speed. When the rotationspeed of the engine 21 is fixed, the vehicle speed changes in responseto the rotation speed ratio of the power transmission device 24. Therotation speed ratio is the ratio of the rotation speed of the outputshaft 63 with respect to the rotation speed of the input shaft 61.Therefore, the variation in the vehicle speed in FIG. 4 matches thevariation of the rotation speed ratio of the power transmission device24. That is, FIG. 4 illustrates the relationship between the rotationspeeds of the motors MG1 and MG2 and the rotation speed ratio of thepower transmission device 24. The solid line in FIG. 4 represents therotation speed of the first motor MG1, and the dashed line representsthe rotation speed of the second motor MG2.

The L-clutch CL is connected and the H-clutch CH is disconnected in theregion which the vehicle speed is between zero and V1 inclusive (Lomode). Because the H-clutch CH is disconnected in the Lo mode, thesecond carrier C2 and the first ring gear R1 are disconnected. Becausethe L-clutch CL is connected, the second carrier C2 is fixed.

The driving power from the engine 21 in the Lo mode is inputted to thefirst sun gear S1 via the transmission shaft 67, and the driving poweris outputted from the first carrier C1 to the second sun gear S2.Conversely, the driving power inputted to the first sun gear S1 istransmitted from the first planet gears P1 to the first ring gear R1 andoutputted through the first ring outer periphery gear Gr1 and the secondmotor gear Gm2 to the second motor MG2. The second motor MG2 functionsmainly as a generator in the Lo mode, and a portion of the electricalpower generated by the second motor MG2 is stored in the capacitor 64. Aportion of the electrical power generated by the second motor MG2 isconsumed in the driving of the first motor MG1.

The first motor MG1 functions mainly as an electric motor in the Lomode. The driving power of the first motor MG1 is outputted to thesecond sun gear S2 along a path from the first motor gear Gm1 to thefirst carrier gear Gc1 to the first carrier C1. The driving poweroutputted to the second sun gear S2 as described above is transmitted tothe output shaft 63 along a path from the second planet gears P2 to thesecond ring gear R2 to the second ring outer periphery gear Gr2 to theoutput gear 71.

The H-clutch CH is connected and the L-clutch CL is disconnected in theregion in which the vehicle speed exceeds V1 (Hi mode). Because theH-clutch CH is connected in the Hi mode, the second carrier C2 and thefirst ring gear R1 are connected. Because the L-clutch CL isdisconnected, the second carrier C2 is disconnected. Therefore, therotation speeds of the first ring gear R1 and the second carrier C2match.

The driving power from the engine 21 in the Hi mode is inputted to thefirst sun gear S1 and the driving power is outputted from the firstcarrier C1 to the second sun gear S2. The driving power inputted to thefirst sun gear S1 is outputted from the first carrier C1 through thefirst carrier gear Gc1 and the first motor gear Gm1 to the first motorMG1. The first motor MG1 functions mainly as a generator in the Hi mode,and thus a portion of the electrical power generated by the first motorMG1 is stored in the capacitor 64. A portion of the electrical powergenerated by the first motor MG1 is consumed in the driving of thesecond motor MG2.

The driving power of the second motor MG2 is outputted to the secondcarrier C2 along a path from the second motor gear Gm2 to the first ringouter periphery gear Gr1 to the first ring gear R1 to the H-clutch CH.The driving power outputted to the second sun gear S2 as described aboveis outputted through the second planet gears P2 to the second ring gearR2, and the driving power outputted to the second carrier C2 isoutputted through the second planet gears P2 to the second ring gear R2.The driving power combined by the second ring gear R2 in this way istransmitted through the second ring outer periphery gear Gr2 and theoutput gear 71 to the output shaft 63.

While forward travel driving has been discussed above, the operations ofreverse travel driving are the same. During braking, the roles of thefirst motor MG1 and the second motor MG2 as generator and motor arereversed from the above explanation.

The control of the power transmission device 24 by the control unit 27is described in detail below. The control unit 27 controls the outputtorque of the power transmission device 24 by controlling the motortorque of the first motor MG1 and the second motor MG2. That is, thecontrol unit 27 controls the tractive force of the work vehicle 1 bycontrolling the motor torque of the first motor MG1 and the second motorMG2. A method for determining the command values (referred to below as“command torques”) of the motor torques to the first motor MG1 and thesecond motor MG2 is explained below.

FIG. 5 is a control block diagram illustrating processing executed bythe control unit 27. The control unit 27 has a transmission requirementdetermination unit 84, an energy management requirement determinationunit 85, and a work implement requirement determination unit 86 asillustrated in FIG. 5.

The transmission requirement determination unit 84 determines a requiredtractive force Tout on the basis of an accelerator operation amount Aacand an output rotation speed Nout. Specifically, the transmissionrequirement determination unit 84 determines the required tractive forceTout from the output rotation speed Nout on the basis of requiredtractive force characteristics information D1 stored in the storage unit56. The required tractive force characteristics information D1 is dataindicating the required tractive force characteristics for defining therelationship between the output rotation speed Nout and the requiredtractive force Tout.

Specifically as illustrated in FIG. 6, the storage unit 56 stores dataLout1 (referred to below as “basic tractive force characteristicsLout1”) indicating basic required tractive force characteristics. Thebasic tractive force characteristics Lout1 are required tractive forcecharacteristics when the accelerator operation amount Aac is at themaximum value, that is, at 100%. The basic tractive forcecharacteristics Lout1 are determined in response to a speed rangeselected with the speed change operating member 53 a. The transmissionrequirement determination unit 84 determines current required tractiveforce characteristics Lout2 by multiplying a tractive force ratio FWRand a vehicle speed ratio VR by the basic tractive force characteristicsLout1.

The storage unit 56 stores tractive force ratio information D2 andvehicle speed ratio information D3. The tractive force ratio informationD2 defines the tractive force ratio FWR with respect to the acceleratoroperation amount Aac. The vehicle speed ratio information D3 defines thevehicle speed ratio VR with respect to the accelerator operation amountAac. The transmission requirement determination unit 84 determines thetractive force ratio FWR and the vehicle speed ratio VR in response tothe accelerator operation amount Aac. The transmission requirementdetermination unit 84 determines the current required tractive forcecharacteristics Lout2 in response to the accelerator operation amountAac by multiplying the basic tractive force characteristics Lout1 by thetractive force ratio FWR in the vertical axis direction which indicatesthe required tractive force and the vehicle speed ratio VR in thehorizontal axis direction which indicates the output rotation speedNout.

The tractive force ratio information D2 defines the tractive force ratioFWR that increases in correspondence to an increase in the acceleratoroperation amount Aac. The vehicle speed ratio information D3 defines thevehicle speed ratio VR which increases in correspondence to an increasein the accelerator operation amount Aac. However, the tractive forceratio FWR is greater than zero when the accelerator operation amount iszero. Similarly, the vehicle speed ratio VR is greater than zero whenthe accelerator operation amount Aac is zero. As a result, the requiredtractive force Tout is a value greater than zero even when theaccelerator operating member 51 a is not being operated. That is,tractive force is being outputted from the power transmission device 24even when the accelerator operating member 51 a is not being operated.As a result, a behavior similar to creep generated in a torqueconverter-type speed change device is materialized in the EMT-type powertransmission device 24.

The required tractive force characteristics information D1 defines therequired tractive force Tout that increases in response to a reductionin the output rotation speed Nout. When the above-mentioned speed changeoperating member 53 a is operated, the transmission requirementdetermination unit 84 changes the required tractive forcecharacteristics in response to the speed range selected by the speedchange operating member 53 a. For example, when a down-shift isconducted using the speed change operating member 53 a, the requiredtractive force characteristics information changes from Lout2 to Lout2′as illustrated in FIG. 6. As a result, the upper limit of the outputrotation speed Nout is reduced. That is, the upper limit of the vehiclespeed is reduced.

The required tractive force characteristics information D1 defines therequired tractive force Tout as a negative value with respect to theoutput rotation speed Nout that is no less than the predetermined speed.As a result, the required tractive force Tout is determined to be anegative value when the output rotation speed Nout is larger than theupper limit of the output rotation speed in the selected speed range. Abraking force is generated when the required tractive force Tout is anegative value. As a result, a behavior similar to engine brakegenerated in a torque converter-type speed change device is materializedin the EMT-type power transmission device 24.

The energy management requirement determination unit 85 illustrated inFIG. 5 determines an energy management required horsepower Hem on thebasis of a remaining amount of electrical power in the capacitor 64. Theenergy management required horsepower Hem is a horsepower required bythe power transmission device 24 for charging the capacitor 64. Theenergy management requirement determination unit 85 determines a currentcapacitor capacity from a voltage Vca of the capacitor 64. The energymanagement requirement determination unit 85 increases the energymanagement required horsepower Hem as the current capacitor capacitybecomes smaller.

The work implement requirement determination unit 86 determines a workimplement required horsepower Hpto on the basis of a work implement pumppressure Pwp and an operation amount Awo (referred to below as “workimplement operation amount Awo”) of the work implement operating member52 a. In the present exemplary embodiment, the work implement requiredhorsepower Hpto is a horsepower distributed to the work implement pump23. However, the work implement required horsepower Hpto may include ahorsepower distributed to the steering pump 30 and/or the transmissionpump 29.

Specifically, the work implement requirement determination unit 86determines a required flow rate Qdm of the work implement pump 23 fromthe work implement operation amount Awo on the basis of required flowrate information D4. The required flow rate information D4 is stored inthe storage unit 56 and defines the relationship between the requiredflow rate Qdm and the work implement operation amount Awo. The requiredflow rate information D4 defines the relationship between the requiredflow rate Qdm and the work implement operation amount Awo so that therequired flow rate Qdm increases in correspondence with an increase inthe work implement operation amount Awo. The work implement requirementdetermination unit 86 determines the work implement required horsepowerHpto from the required flow rate Qdm and the work implement pumppressure Pwp.

Furthermore, the work implement requirement determination unit 86determines a work implement required engine rotation speed Nedm on thebasis of the required flow rate Qdm and the discharge capacity of thework implement pump 23. Specifically, the work implement requirementdetermination unit 86 determines a work implement required pump rotationspeed by dividing the required flow rate Qdm by the discharge capacityof the work implement pump 23. The work implement requirementdetermination unit 86 then determines the work implement required enginerotation speed Nedm from the work implement required pump rotation speedin consideration of the factors such as the number of teeth of therotating elements and the transmission efficiency between the engine 21and the work implement pump 23.

The control unit 27 has a target output shaft torque determining unit82, a target input shaft torque determining unit 81, and acommand-torque determination unit 83.

The target output shaft torque determining unit 82 determines a targetoutput shaft torque To_ref. The target output shaft torque To_ref is atarget value for the torque to be outputted from the power transmissiondevice 24. The target output shaft torque determining unit 82 determinesthe target output shaft torque To_ref on the basis of the requiredtractive force Tout determined by the transmission requirementdetermination unit 84. That is, the target output torque To_ref isdetermined so that the tractive force outputted from the powertransmission device 24 obeys the required tractive force characteristicsdefined by the force characteristics information D1. Specifically, thetarget output shaft torque To_ref is determined by multiplying therequired tractive force Tout by a predetermined distribution ratio. Thepredetermined distribution ratio is set, for example, so that the totalof the work implement required horsepower Hpto, the transmissionrequired horsepower Htm, and the energy management required horsepowerHem does not exceed the output horsepower from the engine 21.

The target input shaft torque determining unit 81 determines a targetinput shaft torque Te_ref. The target input shaft torque Te_ref is atarget value for the torque to be inputted to the power transmissiondevice 24. The target input shaft torque determining unit 81 determinesthe target input shaft torque Te_ref on the basis of the transmissionrequired horsepower Htm and the energy management required horsepowerHem. Specifically, the target input shaft torque determining unit 81calculates the target input shaft torque Te_ref by multiplying theengine rotation speed by the sum of the energy management requiredhorsepower Hem and the value of the transmission required horsepower Htmmultiplied by the predetermined distribution ratio. The transmissionrequired horsepower Htm is calculated by multiplying the above-mentionedrequired tractive force Tout by the current output rotation speed Nout.

The command-torque determination unit 83 uses torque-balance informationto determine command torques Tm1_ref and Tm2_ref to the motors MG1 andMG2 from the target input shaft torque Te_ref and the target outputshaft torque To_ref. The torque-balance information defines arelationship between the target input shaft torque Te_ref and the targetoutput shaft torque To_ref to achieve a balance among the torques of thepower transmission device 24. The torque-balance information is storedin the storage unit 56.

As described above, the transmission paths of the driving power in thepower transmission device 24 are different for the Lo mode and the Himode. As a result, the command-torque determination unit 83 usesdifferent torque-balance information to determine the command torquesTm1_ref and Tm2_ref for the motors MG1 and MG2 in the Lo mode and the Himode. Specifically, the command-torque determination unit 83 uses firsttorque-balance information represented by equation 1 below to determinecommand torques Tm1_Low and Tm2_Low for the motors MG1 and MG2 in the Lomode. In the present exemplary embodiment, the first torque-balanceinformation is an equation for balancing the torques of the powertransmission device 24.

(Equation 1)

Ts1_Low=Te_ref*r_fr

Tc1_Low=Ts1_Low*(−1)*((Zr1/Zs1)+1)

Tr2_Low=To_ref*(Zod/Zo)

Ts2_Low=Tr2_Low*(Zs2/Zr2)

Tcp1_Low=Tc1_Low+Ts2_Low

Tm1_Low=Tcp1_Low*(−1)*(Zp1/Zp1d)

Tr1_Low=Ts1_Low*(Zr1/Zs1)

Tm2_Low=Tr1_Low*(−1)*(Zp2/Zp2d)

The command-torque determination unit 83 uses second torque-balanceinformation represented by equation 2 below to determine command torquesTm1_Hi and Tm2_Hi for the motors MG1 and MG2 in the Hi mode. In thepresent exemplary embodiment, the second torque-balance information isan equation for balancing the torques of the power transmission device24.

ti (Equation 2)

Ts1_Hi=Te_ref*r_fr

Tc1_Hi=Ts1_Hi*(−1)*((Zr1/Zs1)+1)

Tr2_Hi=To_ref*(Zod/Zo)

Ts2_Hi=Tr2_Hi*(Zs2/Zr2)

Tcp1_Hi=Tc1_Hi+Ts2_Hi

Tm1_Hi=Tcp1_Hi*(−1)*(Zp1/Zp1d)

Tr1_Hi=Ts1_Hi*(Zr1/Zs1)

Tc2_Hi=Tr2_Hi*(−1)*((Zs2/Zr2)+1)

Tcp2_Hi=Tr1_Hi+Tc2_Hi

Tm2_Hi=Tcp2_Hi*(−1)*(Zp2/Zp2d)

The contents of the parameters in each torque-balance information aredepicted in Table 1 below.

TABLE 1 Te_ref Target input shaft torque To_ref Target output shafttorque r_fr Deceleration ratio for the FR switch mechanism 65 (The FRswitch mechanism 65 decelerates the engine rotation speed at l/r_fr andoutput it. When the FR switch mechanism 65 is in the forward travelstate, r_fr is a negative value. When the FR switch mechanism 65 is inthe reverse travel state, r_fr is a positive value.) Zs1 Number of teethof the sun gear S1 in the first planetary gear mechanism 68. Zr1 Numberof teeth of the ring gear R1 in the first planetary gear mechanism 68.Zp1 Number of teeth in the first carrier gear Gc1 Zp1d Number of teethof the first motor gear Gm1 Zs2 Number of teeth of the sun gear S2 inthe second planetary gear mechanism 69. Zr2 Number of teeth of the ringgear R2 in the second planetary gear mechanism 69. Zp2 Number of teethof the first ring outer periphery gear Gr1 Zp2d Number of teeth of thesecond motor gear Gm2 Zo Number of teeth of the second ring outerperiphery gear Gr2 Zod Number of teeth of the output gear 71

The control of the engine 21 by the control unit 27 is described indetail below. As described above, the control unit 27 controls theengine by transmitting command signals to the fuel injection device 28.A method for determining the command throttle values for the fuelinjection device 28 will be explained below. The control unit 27 has anengine requirement determination unit 87 and a required throttledetermination unit 89.

The engine requirement determination unit 87 determines an enginerequired horsepower Hdm on the basis of the work implement requiredhorsepower Hpto, the transmission required horsepower Htm, and theenergy management required horsepower Hem. Specifically, the enginerequirement determination unit 87 determines the engine requiredhorsepower Hdm by adding the work implement required horsepower Hpto,the transmission required horsepower Htm, and the energy managementrequired horsepower Hem.

The required throttle determination unit 89 determines a commandthrottle value Th_cm from the engine required horsepower Hdm, theaccelerator operation amount Aac, and the work implement required enginerotation speed Nedm. Specifically, the storage unit 56 stores an enginetorque line Let and a matching line Lma as illustrated in FIG. 7. Theengine torque line Let defines a relationship between the output torqueof the engine 21 and the engine rotation speed Ne. The engine torqueline Let includes a regulation region La and a full load region Lb. Theregulation region La changes in response to the command throttle valueTh_cm (see La′ in FIG. 7). The full load region Lb includes a ratedpoint Pr and a maximum torque point Pm located on the low enginerotation speed side from the rated point Pr.

The matching line Lma is information for determining a first requiredthrottle value Th_tm1 from the engine required horsepower Hdm. While thematching line Lma can be set optionally, the matching line Lma in thepresent embodiment is set so as to pass through a position closer to themaximum torque point Pm than the rated point Pr in the full load regionLb of the engine torque line Let.

The required throttle determination unit 89 determines the firstrequired throttle value Th_tm1 so that the engine torque line Let andthe matching line Lma match at a matching point Pm1 where the outputtorque of the engine 21 becomes the torque corresponding to the enginerequired horsepower Hdm. That is, the intersection of the matching lineLma and an equivalent horsepower line Lhdm corresponding to the enginerequired horsepower Hdm is set as a first matching point Pma1, and therequired throttle determination unit 89 determines the first requiredthrottle value Th_tm1 so that the regulation region (see “La”) of theengine torque line Let passes through the first matching point Pma1.

The required throttle determination unit 89 determines the lowest of thefirst required throttle value Th_tm1 and a second required throttlevalue Th_tm2 corresponding to the accelerator operation amount Aac, as athird command throttle value Th_tm3. Moreover, when a below-mentionedspeed control of the work implement 3 due to the engine rotation speedis performed, the required throttle determination unit 89 determines afourth required throttle value Th_tm4 on the basis of the work implementrequired engine rotation speed Nedm. Specifically, the required throttledetermination unit 89 determines the fourth required throttle valueTh_tm4 so that the regulation region (see La″) of the engine torque lineLet passes through a point Pma2 in which the engine rotation speedbecomes the work implement required engine rotation speed Nedm on theequivalent horsepower line Lhdm. The required throttle determinationunit 89 determines the largest of the third command throttle valueTh_tm3 and the fourth required throttle value Th_tm4 as the commandthrottle value Th_cm. When the speed control of the work implement 3 isnot performed using the engine rotation speed, the required throttledetermination unit 89 determines the third command throttle value Th_tm3as the command throttle value Th_cm.

The following is an explanation of the speed control of the workimplement 3. FIG. 8 is a graph illustrating a relationship between thework implement operation amount and the discharge flow rate of the workimplement pump 23. The speed of the work implement 3 increases incorrespondence to an increase in the discharge flow rate of the workimplement pump 23. Therefore, changes in the discharge flow rate of thework implement pump 23 in FIG. 8 illustrate changes in the speed of thework implement 3. The relationship between the work implement operationamount and the discharge flow rate of the work implement pump may bemodified and is not necessarily the linear shape depicted in FIG. 8.

As illustrated in FIG. 8, the control unit 27 causes the discharge flowrate of the work implement pump 23 to be increased in response to anincrease in the work implement operation amount. The control unit 27controls the opening surface area of the work implement control valve 41by determining the command current value for the work implement controlvalve 41 in response to the work implement operation amount. Asdescribed above, the first capacity control device 42 uses the LS valve46 to control the discharge capacity of the work implement pump 23 sothat the differential pressure between the discharge pressure of thework implement pump 23 and the outlet hydraulic pressure of the workimplement control valve 41 becomes a predetermined value. When the workimplement operation amount is zero or greater but less than a1 in FIG.8, the discharge capacity of the work implement pump 23 increases inresponse to an increase in the work implement operation amount wherebythe discharge flow rate of the work implement pump 23 increases. Thatis, the speed of the work implement 3 is controlled due to the dischargecapacity of the work implement pump 23 being controlled.

When the work implement operation amount reaches a1, the dischargecapacity of the work implement pump 23 becomes the maximum capacity.When the work implement operation amount reaches or exceeds a1, thecontrol unit 27 determines the command throttle value Th_cm on the basisof the work implement required engine rotation speed Nedm. That is, whenthe work implement operation amount is equal to or greater than a1, therequired throttle determination unit 89 causes the engine rotation speedto be increased in response to the increase in the operation amount ofthe work implement operating member 52 a. As a result, the speed of thework implement 3 increases. The discharge flow rate is fixed at an upperlimit Qmax when the work implement operation amount is equal to orgreater than a2.

When the speed control of the work implement 3 is performed using theengine rotation speed as described above, the transmission requirementdetermination unit 84 causes the required tractive force to fall to avalue lower than a value determined on the basis of the operation amountof the accelerator operating member 51 a. Specifically as illustrated inFIG. 6, the transmission requirement determination unit 84 causes therequired tractive force to be reduced by multiplying the vehicle speedratio VR by a predetermined reduction rate. The predetermined reductionrate is a value less than one. The predetermined reduction rate is setso as to increase in response to an increase in the work implementoperation amount. Alternatively, the predetermined reduction rate may bea fixed value.

FIGS. 9A-9E are timing charts illustrating changes in parameters whencontrolling the speed of the work implement 3. As illustrated in FIG.9B, the accelerator operation amount is fixed at Aac1. When the workimplement operation amount is zero (point in time 0 to t1) asillustrated in FIG. 9A, the speed of the work implement 3 is zero asillustrated in FIG. 9D. The engine rotation speed is fixed at Ne1 asillustrated in FIG. 9C and the tractive force is fixed at F1 asillustrated in FIG. 9E.

When the work implement operation amount increases from zero, the speedof the work implement 3 is controlled by controlling the dischargecapacity until the discharge capacity of the work implement 3 reachesthe maximum capacity. As a result, while the engine rotation speed isfixed at Ne1, the speed of the work implement 3 increases (point in timet1 to t2). The tractive force is also fixed at F1 at this time.

When the work implement operation amount increases further and thedischarge capacity of the work implement 3 reaches the maximum capacity,the speed of the work implement 3 is controlled by controlling theengine rotation speed (point in time t2 to t3). The engine rotationspeed increases and the speed of the work implement 3 increases incorrespondence to the increase in the work implement operation amount.Moreover, the tractive force is reduced in correspondence to theincrease in the work implement operation amount.

When the work implement operation amount meets or exceeds a2, the enginerotation speed is fixed at Ne2. The speed of the work implement 3 isfixed at V1. The tractive force is fixed at F2.

The work vehicle 1 according to the present exemplary embodiment has thefollowing features.

When the operator operates the work implement operating member 52 a, theengine rotation speed is increased automatically due to the speedcontrol of the work implement 3. As a result, the operator is able toadjust the speed of the work implement 3 by operating the work implementoperating member 52 a without operating the accelerator operating member51 a.

Moreover, the command torques for the motors MG1 and MG2 are determinedso that the tractive force of the work vehicle 1 becomes the tractiveforce determined with the required tractive force characteristics. As aresult, an increase in the tractive force is suppressed even if theengine rotation speed is increased due to the speed control of the workimplement 3. Therefore, an increase in the vehicle speed can besuppressed without the operator operating the brake operating member 58a. As described above, the work vehicle 1 according to the presentexemplary embodiment allows for the adjustment of the speed of the workimplement 3 and the vehicle speed with an easy operation.

The required tractive force is determined on the basis of theaccelerator operation amount. Moreover, the command torques for themotors MG1 and MG2 are determined so that the tractive force of the workvehicle 1 becomes the required tractive force. Therefore, the tractiveforce is outputted in accordance with the accelerator operation amount.As a result, the vehicle speed can be adjusted in response to theaccelerator operation amount even if the engine rotation speed increasesdue to the speed control of the work implement 3.

The speed of the work implement 3 is controlled by controlling thedischarge capacity of the work implement pump 23 until the dischargecapacity becomes the maximum capacity. The speed of the work implement 3is controlled with the engine rotation speed when the discharge capacityof the work implement pump 23 reaches the maximum capacity. Accordingly,fuel consumption can be improved.

The speed of the work implement 3 when the work implement operatingmember 52 a is operated to the predetermined amount while the workvehicle 1 is traveling at a high engine rotation speed is the same asthe speed of the work implement 3 when the work implement operatingmember 52 a is operated to the same amount as the above predeterminedamount while the work vehicle 1 is traveling at a low engine rotationspeed. That is, the relationship between the work implement operationamount and the speed of the work implement 3 is the same regardless ofwhether the engine rotation speed is high or low.

The required tractive force is reduced to a value lower than a valuedetermined on the basis of the accelerator operation amount when thespeed control of the work implement 3 is performed using the enginerotation speed. In this case, a behavior similar to a conventional workvehicle can be realized. That is, the driving power distributed to thetravel device 25 is reduced by increasing the driving power distributedto the work implement pump 23 when the operator operates the workimplement operating member 52 a to increase the speed of the workimplement 3 in the conventional work vehicle. As a result, the behaviorof a reduction in the tractive force is brought about in the vehiclewhen the operator operates the work implement operating member 52 a. Asense of discomfort by the operator can be suppressed in the workvehicle 1 according to the present exemplary embodiment due to therealization of the behavior similar to the conventional work vehicle inthis way.

The present invention is not limited to the above exemplary embodimentsand various changes and modifications may be made without departing fromthe spirit of the invention.

The present invention is not limited to the above-mentioned wheel loaderand may be applied to another type of work vehicle, such as a bulldozer,a tractor, a forklift, or a motor grader.

The present invention may be applicable to another type of speed changedevice, such as an HMT, without being limited to the EMT. In this case,the first motor MG1 functions as a hydraulic motor and a hydraulic pump.The second motor MG2 functions as a hydraulic motor and a hydraulicpump. The first motor MG1 and the second motor MG2 are variablecapacitor pump/motors, and the capacities are controlled by the controlunit 27 controlling the tilt angle of the skew plate or the inclinedshaft. The capacities of the first motor MG1 and the second motor MG2are controlled so that the command torques Tm1_ref and Tm2_refcalculated in the same way as in the above exemplary embodiments areoutputted.

The configuration of the power transmission device 24 is not limited tothe configuration of the above exemplary embodiments. For example, thecoupling and disposition of the elements of the two planetary gearmechanisms 68 and 69 are not limited to the coupling and disposition ofthe above exemplary embodiments. However, the number of the planetarygear mechanisms provided in the power transmission device 24 is notlimited to two. The power transmission device 24 may only have oneplanetary gear mechanism. Alternatively, the power transmission device24 may have three or more planetary gear mechanisms.

The control of the power transmission device 24 is not limited to thecontrol of the above exemplary embodiment. That is in the presentexemplary embodiment, the target input shaft torque Te_ref and thetarget output shaft torque To_ref are determined so that predeterminedvehicle speed—tractive force characteristics can be achieved in whichthe tractive force changes continuously in response to the vehiclespeed. However, the target input shaft torque Te_ref and the targetoutput shaft torque To_ref may be set optionally.

The torque-balance information is not limited to the equations forbalancing the torque as in the above exemplary embodiment. For example,the torque-balance information may be in the format of a table or a map.

The work implement pump is not limited to one and two or more workimplement pumps may be provided. In this case, the above-mentioneddischarge capacity is the sum of the discharge capacities of theplurality of work implement pumps.

The reduction of the tractive force may not be performed when the speedcontrol of the work implement 3 is performed by controlling the enginerotation speed. Alternatively, the reduction of the tractive force isnot limited to the method of multiplying the vehicle speed ratio VR by apredetermined reduction rate, and another method may be used.

The tractive force can be increased slightly as depicted by the dashedline in FIG. 10E due to inertia of the planetary gear mechanismconnected to the engine when the engine rotation speed is increased onthe basis of a lever operation in the work vehicle provided with theplanetary gear mechanism as in the above exemplary embodiment.Therefore, the tractive force is greater in comparison to when a speedcontrol is not performed when the engine rotation speed is increased dueto speed control of the work implement 3 being performed by controllingthe engine rotation speed as described above. This increase in thetractive force is tied to an increase in the traveling acceleration ofthe work vehicle and leads to an increase in the vehicle speed of thework vehicle. A sense of discomfort may be felt by the operator due tothe increase in the vehicle speed brought about by an operation of thework implement lever in this way.

Accordingly, when the speed control of the work implement 3 is performedby controlling the engine rotation speed, the reduction rate may bedetermined so that the tractive force is maintained regardless of theoperation of the work implement operating member 52 a. In this case, therequired tractive force is reduced so that the tractive force ismaintained regardless of the operation amount of the work implementoperating member 52 a even if the engine rotation speed increases due tothe above control. As a result, the tractive force is maintained at thelevel before the operation of the work implement operating member 52 aas depicted by the solid line in FIG. 10E. FIGS. 10A-10D are similar toFIGS. 9A-9D, respectively. As a result, any sense of discomfort felt bythe operator can be suppressed.

The speed control of the work implement 3 may be performed bycontrolling the engine rotation speed before the capacity of the workimplement pump 23 reaches the maximum capacity.

The power transmission device is not limited to a so-called split systemdevice using the planetary gear mechanism as described above, and mayuse a device of another system. For example, FIG. 11 is a schematic viewillustrating a power transmission device 124 according to a firstmodified example. The power transmission device 124 illustrated in FIG.11 is a so-called series system power transmission device. The engine 21in the power transmission device 124 only uses the first motor MG1 togenerate electricity. The second motor MG2 uses the electrical powergenerated in the first motor MG1 to drive the travel device. The secondmotor MG2 also generates electricity by recovering energy duringdeceleration.

Alternatively, the power transmission device is not limited to aso-called hybrid power transmission device that uses the motors asdescribed above. For example, FIG. 12 is a schematic view illustrating apower transmission device 324 according to a second modified example.The power transmission device 324 is a so-called hydro-statictransmission (HST) device. The power transmission device 324 has atravel pump 301 and a travel motor 302. The travel pump 301 is driven bythe engine 21. The travel pump 301 is a variable capacity hydraulic pumpand the capacity of the travel pump 301 is controlled by a pump capacitycontrol device 303. The travel motor 302 drives the travel device bybeing driven by hydraulic fluid discharged from the travel pump 301. Thetravel motor 302 is a variable capacity hydraulic motor and the capacityof the travel motor 302 is controlled by a motor capacity control device304. The vehicle speed and the tractive force are controlled bycontrolling the engine rotation speed, the capacity of the travel pump301, and the capacity of the travel motor 302 and the like.

Configurations that are the same in the above exemplary embodiment areprovided with the same reference numerals in FIGS. 11 and 12 andexplanations thereof are omitted.

A work vehicle that is able to adjust the speed of the work implementwith a simple operation, and a method for controlling the work vehicleare provided according to exemplary embodiments of the presentinvention.

1. A work vehicle comprising: an engine; a hydraulic pump driven by theengine; a work implement driven by hydraulic fluid discharged from thehydraulic pump; a travel device driven by the engine; an acceleratoroperating member configured to change an engine rotation speed; a workimplement operating member configured to operate the work implement; anda control unit configured to increase a speed of the work implement byincreasing the engine rotation speed when an operation amount of thework implement operating member is increased.
 2. The work vehicleaccording to claim 1 further comprising a power transmission deviceincluding an input shaft, an output shaft, and a motor, the powertransmission device transmitting driving power from the engine to thetravel device; the power transmission device being configured so that arotation speed ratio of the output shaft with respect to the input shaftis changed by changing a rotation speed of the motor, and the controlunit controlling a tractive force of the vehicle by controlling anoutput torque of the motor.
 3. The work vehicle according to claim 2,wherein the control unit determines a required tractive force which is atarget tractive force of the travel device on the basis of the operationamount of the accelerator operating member and controls the outputtorque of the motor so that the tractive force of the vehicle becomesthe required tractive force.
 4. The work vehicle according to claim 1,wherein the control unit causes a discharge capacity of the hydraulicpump to increase in response to an increase of the operation amount ofthe work implement operating member when the operation amount of thework implement operating member is equal to or less than a predeterminedoperation amount, the discharge capacity of the hydraulic pump becomes amaximum capacity when the operation amount of the work implementoperating member is the predetermined operation amount, and the controlunit causes the engine rotation speed to be increased in response to anincrease in the operation amount of the work implement operating memberwhen the operation amount of the work implement operating member isgreater than the predetermined operation amount.
 5. The work vehicleaccording to claim 1 wherein the control unit has a storage unitconfigured to store required flow rate information which defines arelationship between the operation amount of the work implementoperating member and a required flow rate to the hydraulic pump, thecontrol unit determines the required flow rate corresponding to theoperation amount of the work implement operating member by referring tothe required flow rate information, and the control unit determines theengine rotation speed on the basis of the required flow rate and thedischarge capacity of the hydraulic pump.
 6. The work vehicle accordingto claim 5, further comprising a work implement control valve configuredto control a hydraulic pressure supplied to the work implement; and acapacity control device having a load sensing valve and controlling thedischarge capacity of the hydraulic pump so that a differential pressurebetween a discharge pressure of the hydraulic pump and an outlethydraulic pressure of the work implement control valve becomes apredetermined value.
 7. The work vehicle according to claim 3, whereinthe control unit causes the required tractive force to fall to a valuelower than a value determined on the basis of the operation amount ofthe accelerator operating member when the engine rotation speed isincreased in response to an increase in the operation amount of the workimplement operating member.
 8. The work vehicle according to claim 7,wherein the control unit causes the required tractive force to bereduced so that the tractive force is reduced in comparison to atractive force before the operation of the work implement operatingmember.
 9. The work vehicle according to claim 7, wherein the powertransmission device has a planetary gear mechanism, and the control unitcauses the required tractive force to be reduced so that the tractiveforce is maintained regardless of the operation amount of the workimplement operating member.
 10. The work vehicle according to claim 7wherein the control unit causes the required tractive force to bereduced by multiplying the required tractive force by a predeterminedreduction rate.
 11. The work vehicle according to claim 1, furthercomprising a power transmission device including an input shaft, anoutput shaft, and a motor, the power transmission device transmittingdriving power from the engine to the travel device; a work implementcontrol valve configured to control a hydraulic pressure supplied to thework implement; and a capacity control device having a load sensingvalve and controlling the discharge capacity of the hydraulic pump sothat a differential pressure between a discharge pressure of thehydraulic pump and an outlet hydraulic pressure of the work implementcontrol valve becomes a predetermined value, the power transmissiondevice being configured so that a rotation speed ratio of the outputshaft with respect to the input shaft is changed by changing a rotationspeed of the motor, the control unit controlling a tractive force of thevehicle by controlling an output torque of the motor, the control unitdetermining a required tractive force which is a target tractive forceof the travel device on the basis of the operation amount of theaccelerator operating member and controls the output torque of the motorso that the tractive force of the vehicle becomes the required tractiveforce, the control unit including a storage unit configured to storerequired flow rate information which defines a relationship between theoperation amount of the work implement operating member and a requiredflow rate to the hydraulic pump, the control unit determining therequired flow rate corresponding to the operation amount of the workimplement operating member by referring to the required flow rateinformation, and the control unit determining the engine rotation speedon the basis of the required flow rate and the discharge capacity of thehydraulic pump.
 12. The work vehicle according to claim 1, furthercomprising a power transmission device including an input shaft, anoutput shaft, a motor, and a planetary gear mechanism, and transmittingdriving power from the engine to the travel device; the powertransmission device being configured so that a rotation speed ratio ofthe output shaft with respect to the input shaft is changed by changinga rotation speed of the motor, and the control unit controlling atractive force of the vehicle by controlling an output torque of themotor, the control unit determining a required tractive force which is atarget tractive force of the travel device on the basis of the operationamount of the accelerator operating member and controls the outputtorque of the motor so that the tractive force of the vehicle becomesthe required tractive force, the control unit causing the requiredtractive force to be lower than a value determined on the basis of theoperation amount of the accelerator operating member when the enginerotation speed increases in response to an increase in the operationamount of the work implement operating member, and the control unitcausing the required tractive force to be reduced so that the tractiveforce is maintained regardless of the operation amount of the workimplement operating member.
 13. A method for controlling a work vehicleequipped with an engine, a hydraulic pump driven by the engine, a workimplement driven by hydraulic fluid discharged from the hydraulic pump,a travel device driven by the engine, an accelerator operating memberfor changing an engine rotation speed, and a work implement operatingmember for operating the work implement, the method comprising: a stepfor causing a speed of the work implement to increase by causing theengine rotation speed to increase when an operation amount of the workimplement operating member is increased.